The present invention relates generally to damping devices. More specifically, the present invention relates to a fluid filled elastomeric bushing damping device of the type which is used for connecting a vibrating element or assembly, which produces varying types of vibrations, to a rigid support.
The invention will be described particularly in connection with a fluid filled elastomeric damping device of the fluid bushing type, which isolates an internal combustion engine, such as in a vehicle, from the support frame of the engine. However, it should be appreciated that the invention has broader applications and may be used for the absorption of shocks, structural leveling, and energy dissipation in a variety of other environments as well.
In the typical vibration isolating engine mount, a body of natural or synthetic rubber is normally employed. While these elastomeric mounts can be designed to operate in a generally satisfactory manner, such materials inherently have a low coefficient of damping which limits their ability to isolate certain objectionable vibratory inputs to the vehicle, such as those particularly disturbing to a modern lightweight unitized vehicle body and frame construction. An increased damping coefficient is possible by the selection of certain rubber polymers and the use of additives but thus far this technique has proven unsatisfactory because of accompanying adverse effects on other properties of the rubber. Also, an increased damping coefficient produces large damping for all vibratory inputs regardless of frequency or amplitude. This is undesirable in an engine mount particularly when the engine experiences low amplitude, high frequency vibrations.
A major effort is in progress for developing a cost effective means of providing a prescribed and varying amount of damping best suited to damping vibrations of varying frequencies and amplitudes. In this connection, it is well known that for best performance in a hydraulic elastomeric engine mount, damping should be at a maximum at the natural frequency of the mount system It is also desirable that the engine mount be able to handle two distinctly different types of vibrations. More specifically, low frequency vibrations of relatively large amplitude should be damped but in such a way that high frequency vibrations of relatively small amplitude remain relatively undamped but are isolated. Unfortunately, a design for successfully damping high amplitude vibrations, on the order of 0.3 mm or greater, generally will also damp low amplitude vibrations, on the order of 0.1 mm or less. Various schemes have addressed this problem with some success. Many of the schemes are based on the provision of a partition member which is capable of limited free motion between two fluid filled chambers.
One of these devices, for example, is an axial or strut-type damping device which provides two fluid filled chambers in which a partition member permits only limited fluid movement between the chambers. Axial damping devices are, however, complex in design, weigh more, are more expensive to manufacture, and need to be larger in size than bushing-type engine mounts. Engine mounts in the form of bushings are also advantageous over strut-type mounts for safety reasons. In this connection, bushin-type engine mounts better restrain a vehicle engine against movement during a crash than do strut-type mounts. Additionally, bushing-type mounts are advantageous over strut-type mounts since they are better able to damp a rocking motion of the engine. Such a motion is frequently encountered in transaxle mounted engines in front wheel drive vehicles.
Bushing-type damping devices attempting to solve this problem have also been found to be inadequate. More specifically, the conventional bushing desings have found it impossible to successfully handle both high frequency, low amplitude vibrations and low frequency, high amplitude vibrations.
Accordingly, it has been considered desirable to develop a new improved vibration damping device for load carrying and selectively varying damping response to structural agitation which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.